We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360 000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi(+)pi(-)->rho ->mu(+)mu(-) annihilation. The associated space-time averaged rho spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.
First measurement of the rho spectral function in high-energy nuclear collisions
DE FALCO, ALESSANDRO;FLORIS, MICHELE;PUDDU, GIOVANNA;USAI, GIANLUCA;
2006-01-01
Abstract
We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360 000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi(+)pi(-)->rho ->mu(+)mu(-) annihilation. The associated space-time averaged rho spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.
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